The present invention relates to an advanced austenitic stainless steel, more particularly to a structural austenitic stainless steel suited for use in a corrosive environment or a high stress loaded environment, and a method for manufacturing the same.
Among the industrial steel materials, austenitic stainless steel has widely been used as a structural material because of its excellent corrosion resistance and workability. This steel, however, is low in strength in comparison with other types of structural steel. Also, although the austenitic stainless steel has high corrosion resistance, it is rather inferior to other types of steel in use in a specific corrosive environment where pitting or stress corrosion cracking is likely to occur.
With the progress of working efficiency and weight reduction of the products in recent years, request has been rising in the industries for high strength structural materials. In order to comply with such request, attempts have been made to achieve higher strength of structural materials by use of additional alloy elements such as rare metals, but use of such elements is not preferable in view of recycling of the materials. One method for improving strength and corrosion resistance of an alloy without changing the alloy composition is to utilize finer crystal grains.
The primary cause of deformation of metal material is slip deformation caused by the so-called dislocation, which is the transfer of lattice defect existing in the crystal. A high resistance is produced due to the interaction between the grain boundaries and dislocation when dislocation passes across the crystal grain boundaries.
To decrease grain size is to raise the density of the crystal grain boundaries, and the phenomenon of deformation resistance increased by decreasing grain size is well known as the Hall-Petch relationship, i.e., yield stress increases in proportion to the xe2x88x92xc2xd power of the crystal grain size.
For the alloys containing an element capable of forming a protective film, such as Cr, the finer the crystal grains are, the more promoted is the diffusion of the grain boundaries, whereby formation of the protective film is made easier. The impurity element segregated at the grain boundaries is considered as one of the causes of intergranular corrosion, but introduction of the grain boundaries into bulk as a result of grain refinement may dilute the concentration of the impurity element down to improve corrosion resistance. Conventional means for fining the crystal grains of steel materials include thermomechanical treatment comprising combination of such operation as rolling or upsetting with ensuing heat treatment.
Researchers are pursuing studies for comminuting the crystal grains of austenitic stainless steel to the submicron size by thermomechanical treatment making use of deformation-induced martensite transformation and inverse transformation caused at a high temperature. Such studies are reported in, for instance, Tetsu-to-Hagane, The Iron and Steel Institute of Japan, Vol. 80, pp. N529-N535, 1994; and Bulletin of Japan Institute of Metals, Vol. 27, No. 5, pp. 400-402, 1988.
However, as is generally conceived, in a process where a solid soluted material is rolled at a high draft, the crystal grain size is strongly affected by workability, namely by the degree of working in the direction of rolling and in the thickness direction, and tends to have a non-uniform distribution, so that this method is unsuited for obtaining thick-walled components. Further, it is not easy to obtain a high degree of working by cold rolling while avoiding the formation of cracks.
Mechanical milling (mechanical alloying or mechanical grinding), which performs forced working of metal powder by a ball mill or such, is capable of forming powder having a crystal structure of nanometer grain size, since the strain energy accumulated by working is much larger than the conventional methods such as rolling. For consolidating the powder which has undergone mechanical milling (hereinafter referred to as mechanically milled powder), the powder needs to be sintered at a high temperature under a high pressure. Usually, strain energy is released in the course of high-temperature heating to cause coarsening of the crystal grains, so that it is difficult to carry out the consolidation process of the powder while maintaining the nano-scale crystalline state.
Studies are underway for obtaining a bulk material of austenite stainless steel by consolidating its mechanically milled powder and grinding the crystal grains to the submicron size. They are reported, for instance, in (1) JP-A-8-337853, (2) JP-A-10-195502, and (3) Tetsu-to-Hagane, The Iron and Steel Institute of Japan, Vol. 84, pp. 357-362, 1998.
In the materials disclosed in (2) and (3) above, the sigma phase is dispersed to control the growth of austenite crystal grains. However, the M23C6 type carbide or sigma phase, which emerges principally in austenitic stainless steel, is mainly composed of Cr, so that it acts to lower Cr concentration in the surrounding and to encourage corrosion. It is possible to reduce the influence of such carbide or sigma phase by reducing the grain size, but such material can not be deemed suited as dispersed grains to be reduced in size.
The material described in the above literature (1) suggests that precipitation of a carbide or oxide mainly composed of Ti, Zr or Nb is likely to take place, but this literature fails to mention the optimal composition or process conditions for controlling the grain growth.
In the Abstract of the Proceedings of 1998 General Meeting of The Iron and Steel Institute of Japan, Vol. 11, page 563, it is reported that the fine crystal structure of ferrite steel with a nanometer grain size can be stabilized over a high temperature level of 1,000xc2x0 C. or higher by adding and dispersing finely divided Y2O3 with a grain size of several tens nanometers in the ferrite steel. However, when recycling of the material is considered, addition of a specific alloy element such as yttrium in the steel material is undesirable as it may complex the refining process, leading to a rise of production cost.
As viewed above, with the techniques disclosed hitherto, the manufacture of bulk material having a nona-scale ultra-fine crystal structure is possible only under the conditions in which the dimensions and shape of the product are restricted. Also, no disclosure has been made on the optimal compositions or process conditions for achieving high strength and high corrosion resistance.
Accordingly, an object of the present invention is to provide austenitic stainless steel of ultra-fine crystal structure having high strength and high corrosion resistance in comparison with the conventional steel materials, and a method for manufacturing such austenitic stainless steel.
The aspects of the present invention for attaining the above object are as follows.
[1] A corrosion resistant, high strength austenitic stainless steel consisting of 1.0% or less of Si, 2.0% or less of Mn, 0.5% or less of O, 7 to 30% of Ni, 14 to 26% of Cr, 0.3% or less of combination of C and N, at least one element selected from the group consisting of 1.0% or less of Ti, 2.0% or less of Zr and 2.0% or less of Nb, and the balance consisting of Fe and unavoidable impurities, the percentage being given in weight basis; said steel containing carbonitride with a grain size of several to 100 nm dispersed therein;
said steel having an average crystal grain size of 1 xcexcm or less; and
said steel containing 90% by volume or more of austenite phase.
[2] A corrosion resistant, high strength austenitic stainless steel consisting of 1.0% or less of Si, 2.0% or less of Mn, 0.5% or less of O, 7 to 30% of Ni, 14 to 26% of Cr, 3% or less of Mo, 0.3% or less of combination of C and N, at least one element selected from the group consisting of 1.0% or less of Ti, 2.0% or less of Zr and 2.0% or less of Nb, and the balance consisting of Fe and unavoidable impurities, the percentage being given in weight basis;
said steel containing carbonitride with a grain size of several to 100 nm dispersed therein;
said steel having an average crystal grain size of 1 xcexcm or less; and
said steel containing 90% by volume or more of austenite phase.
[3] A method for manufacturing a corrosion resistant, high strength austenitic stainless steel, which comprises the steps of:
providing a mechanically milled powder with an average crystal grain size of 200 nm or less consisting of 1.0% or less of Si, 2.0% or less of Mn, 0.5% or less of O, 7 to 30% of Ni, 14 to 26% of Cr, 0.3% or less of combination of C and N, at least one element selected from the group consisting of 1.0% or less of Ti, 2.0% or less of Zr and 2.0% or less of Nb, and the balance consisting of Fe and unavoidable impurities, the percentage being given in weight basis; and subjecting said mechanically processed powder to a process selected from the group consisting of:
(a) consolidating the mechanically milled powder at 700 to 900xc2x0 C., and
(b) consolidating the mechanically milled powder at 700 to 900xc2x0 C. to obtain a consolidated material and thermomechanically treating the consolidated material.
[4] A method for manufacturing a corrosion resistant, high strength austenitic stainless steel, which comprises the steps of:
providing a mechanically milled powder with an average crystal grain size of 200 nm or less consisting of 1.0% or less of Si, 2.0% or less of Mn, 0.5% or less of O, 7 to 30% of Ni, 14 to 26% of Cr, 3% or less of Mo, 0.3% or less of combination of C and N, at least one element selected from the group consisting of 1.0% or less of Ti, 2.0% or less of Zr and 2.0% or less of Nb, and the balance consisting of Fe and unavoidable impurities, the percentage being given in weight basis; and subjecting said mechanically milled powder to a process selected from the group consisting of:
(a) consolidating the mechanically milled powder at 700 to 900xc2x0 C., and
(b) consolidating the mechanically milled powder at 700 to 900xc2x0 C. to obtain a consolidated material and thermomechanically treating the consolidated material.
In case two or more of Ti, Zr and Nb are used in [1] to [4] described above, the total amount of the two or more of Ti, Zr and Nb is preferably 2.0% or less.
[5] A method for manufacturing a corrosion resistant, high strength austenitic stainless steel as described in [3] or [4] above, wherein the value f determined by the following equation (1) falls within the range of from 0.4 to 2.0:
f=[8.33(C)+7.14(N)]/[1.10(Zr)+2.09(Ti)+1.08(Nb)]xe2x80x83xe2x80x83(1) 
wherein (C), (N), (Ti), (Zr) and (Nb) are the amounts (weights) of the C, N, Ti, Zr and Nb, respectively, in the mechanically milled powder.
[6] A method for manufacturing a corrosion resistant, high strength austenitic stainless steel as described in [1], [2] or [3] above, wherein the value f determined by the following equation (1) falls within the range of from 0.4 to 2.0:
f=[8.33(C)+7.14(N)]/[1.10(Zr+2.09(Ti)+1.08(Nb)]xe2x80x83xe2x80x83(1) 
wherein (C), (N), (Ti), (Zr) and (Nb) are the amounts (weights) of the C, N, Ti, Zr and Nb, respectively, in the mechanically milled powder.
As said mechanically milled powder, it is possible to use products obtained by subjecting a pre-alloy powder or a powder that meets the composition defined above as a whole to mechanical grinding or alloying treatment with an attrition mill or ball mill at 100xc2x0 C. or lower for 30 hours or more so that the products have an average crystal grain size of 200 nm or less.
Preferably, said mechanical grinding or alloying treatment with an attrition mill or ball mill is conducted using steel balls made of an Fe alloy containing 0.3% or less of combination of C and N and having a heat conductivity at 100xc2x0 C. of 16.7 W/mxc2x7K or higher.
Also preferably, said consolidation process are carried out at 700 to 900xc2x0 C. after the mechanically milled powder has been retained at a temperature within a range of from 400 to 650xc2x0 C. for a period of 0.5 to 6 hours, or alternatively, after the mechanically processed powder has been suffered from a rise of temperature from 400 to 650xc2x0 C. for a period of 0.5 to 6 hours.
Preferably, the step of consolidating and the step of successive thermomechanical treatment of the consolidated material include the step of consolidating the mechanically milled powder by hot compression, hot rolling, hot isostatic pressing or hot extrusion at 700 to 900xc2x0 C., or the step of subjecting the consolidated material to a heat treatment or hot forging at 700 to 900xc2x0 C., and the additional step of imparting a desired shape to the consolidated material during any of these preceding steps.
Said corrosion resistant, high strength austenite stainless steel can be worked into a desired shape by press molding at 700 to 900xc2x0 C.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.